Advanced oxidation processes (AOPs) refers to a set of chemical treatment procedures designed to remove organic and sometimes inorganic materials in water and wastewater by oxidation through reactions with hydroxyl radicals (.OH).
AOPs rely on in-situ production of highly reactive hydroxyl radicals (.OH). Hydroxyl radicals are very strong oxidants that can be applied in treatment of water and they can oxidize any compound present in the water matrix, often at a diffusion controlled reaction speed. Consequently, .OH reacts unselectively once formed and contaminants will be quickly and efficiently fragmented and converted into small inorganic molecules. Hydroxyl radicals are produced with the help of one or more primary oxidants (e.g. ozone, hydrogen peroxide, oxygen) and/or energy sources (e.g. ultraviolet light) or catalysts (e.g. titanium dioxide). The present invention applies use of hydrogen peroxide (H2O2) for providing hydroxyl radicals.
In general, when applied in properly tuned conditions, AOPs can reduce the concentration of a contaminant from several hundred ppm to less than 5 ppb and therefore significantly reduce COD and TOC.
The AOP procedure is particularly useful for cleaning biologically toxic or non-degradable materials such as aromatics, pesticides, petroleum constituents, and volatile organic compounds in wastewater. Additionally, AOPs can be used to treat effluent of secondary treated wastewater which is then called tertiary treatment. The contaminant materials are converted to a large extent into stable inorganic compounds such as water, carbon dioxide and salts, i.e. they undergo mineralization. A goal of the wastewater purification by means of AOP procedures is the reduction of chemical contaminants and toxicity to such an extent that the cleaned wastewater may be reintroduced into receiving streams or, at least, into a conventional sewage treatment. More recently, AOPs (e.g., Fenton reaction) have also been used for disinfection of water containing bacteria, fungal and viruses (Gosselin, F., Madeira, L. M., Juhna, T. and Block, J. C. (2013). “Drinking water and biofilm disinfection by Fenton-like reaction”, Water Research 47 (15): 5631-5638).
Although oxidation processes involving .OH have been in use since late 19th century (such as in Fenton reagent), the utilization of such oxidative species in water treatment did not receive adequate attention until Glaze et al. (Glaze, William; Kang, Joon-Wun; Chapin, Douglas H. (1987). “The Chemistry of Water Treatment Processes Involving Ozone, Hydrogen Peroxide and Ultraviolet Radiation.”. Ozone: Science & Engineering: The Journal of the International Ozone Association 9 (4): 335-352) suggested the possible generation of .OH “in sufficient quantity to affect water purification” and defined the term “Advanced Oxidation Processes” for the first time in 1987. AOPs high oxidative capability and efficiency make AOPs an often used technique in tertiary treatment in which the most recalcitrant organic and inorganic contaminants are to be eliminated. The increasing interest in water reuse and more stringent regulations regarding water pollution are currently accelerating the implementation of AOPs at full-scale.
Production of H2O2:
In recent years, Fenton process which involves the in-situ generation of hydroxyl radical (OH.) has provided efficient methods for treatment of recalcitrant organic pollutants. However, the technology is suffering several challenges, among which H2O2 supply and removal of residual H2O2 are two key issues associated with commercial application.
H2O2 as a source of hydroxyl radical (OH.) is often produced through the anthraquinone oxidation and subsequently supplied by dosing in Fenton process, which have inherent problems related to inefficiency and security (See: T. T. Wu and J. D. Englehardt, Environmental Science & Technology, 2012, 46, 2291-2298; S.-J. You, J.-Y. Wang, N.-Q. Ren, X.-H. Wang and J.-N. Zhang, Chemsuschem, 2010, 3, 334-338). Efforts have been made to develop sustainable, efficient and cost-effective H2O2 production technologies.
Electro-Fenton process was developed to in-situ generate H2O2, but the electric energy consumption is relatively high.
Recently, bio-electrochemical systems (BESs) including Microbial electrolysis cells (MECs) and Microbial Fuel cells (MFCs) have been found to be promising as an alternative method for H2O2 production. Microbial Electrolysis Cell (MEC) generates hydrogen or methane from organic material by applying an electric current. The processes require noble-metal-free cathode such as graphite to achieve two-electron reaction [Eq.(1)] and avoid further oxidation of H2O2 (See: L. Fu, S.-J. You, F.-I. Yang, M.-m. Gao, X.-h. Fang and G.-q. Zhang, Journal of Chemical Technology and Biotechnology, 2010, 85, 715-719). The integration of MFCs and Fenton process is called Bioelectro-Fenton and has also been found to provide several advantages over the conventional processes. Nevertheless, the H2O2 production rate of MFCs reported so far is much lower than the expected level with respect to the treatment performance, which may hamper its practical application (See: R. A. Rozendal, E. Leone, J. Keller and K. Rabaey, Electrochemistry Communications, 2009, 11, 1752-1755; O. Modin and K. Fukushi, Water Science and Technology, 2012, 66, 831-836):O2+2e−+2H+→H2O2 (E=+0.450 VSCE)  (1)H2O2+2e−+2H+→H2O (E=+1.534 VSCE)  (2)Reduction of H2O2:
After the Fenton process a residual amount of H2O2 is unavoidable and the residue may cause problems e.g. by causing errors in measurements of biochemical oxygen demand (BOD) and chemical oxygen demand (COD), which measurements are used to establish the content—and reduction—of organic material, or by effecting bacteria activity during subsequent biological treatment.
In lab-scale studies, the content of residual H2O2 reaches from less than 1 mM up to several tens of mM (See: T. T. Wu and J. D. Englehardt, Environmental Science & Technology, 2012, 46, 2291-2298). The residue H2O2 can be account for 70-80% of the H2O2 dose (molar levels) in typical full-scale installation.
Several chemical and physical methods have been developed to remove residual H2O2 or eliminate its interference on COD measurement. However, most of these methods are inefficient, e.g. the methods need additional resources in form of either chemicals or energy such as heating or high pressure, or there is a risk of secondary pollution or the methods might be difficult to monitor.
Thus, an alternative method for removing residual H2O2 in a cost-effective, efficient and environment-friendly way is required.
MFCs are found to fit the requirement very well due to its inherent advantages in anode oxidation and cathode reduction. It is known that H2O2 can be an alternative electron acceptor to oxygen in the Pt-catalyzed cathode of MFCs, where H2O2 is reduced to water [Eq.(2)] (See: B. Tartakovsky and S. R. Guiot, Biotechnol Prog, 2006, 22, 241-246). MFCs could be an ideal technology to remove residual H2O2 as use of MFCs requires no extra chemicals or energy, instead, electricity production is accomplished and electricity production could be considered to be an indicator of the level of residual H2O2.
The Invention:
Compared to MFCs, MECs require a small amount of electricity supply (0.2-0.8 V), but the H2O2 production rate is one to two orders of magnitude higher. Therefore, MECs is considered to be a more suitable partner for Fenton process in view of H2O2 production capacity. No report of MECs based Bioelectro-Fenton system has been available so far.
In order to save electric energy spent on MECs, a renewable and alternative power source is required. MFCs has been used to power MECs for hydrogen production (Sun, M.; Sheng, G. P.; Zhang, L.; Xia, C. R.; Mu, Z. X.; Liu, X. W.; Wang, H. L.; Yu, H. Q.; Qi, R.; Yu, T.; Yang, M., An MEC-MFC-coupled system for biohydrogen production from acetate. Environ Sci Technol 2008, 42, (21), 8095-100). Thus, MFCs therefore also power the MECs for H2O2 production. Furthermore, the concept of using a MFC to remove residual H2O2 from a Fenton process has never been proposed.
The invention therefore relates to a new Bioelectro-Fenton system consisting of both MEC and MFC circuits. In the system of the invention the H2O2 level in the system could be easily controlled by alternately switching between the MEC circuit which is used for H2O2 production and the MFC circuit which is used for removal of residual H2O2.